Stabilization of converted starch pastes



Patented May 21, 1946 Sivert N. Glarum, Ardmore, Pal, and Joseph J. Thomas, Portland, Maine, assignors to Rohm & Haas Company, Philadelphia, Pa.,- 9. comration of Delaware No Drawing. Application October -2o, 1942,

Serial him-462,692

2 Claims.

This invention relates to homogeneous, modified starch pastes which contain converted starch and which are stabilized with a water-soluble carbamide-aldehyde type condensate. This invention further relates to the process of preparing such stabilized modified starch pastes. Specifically, this invention concerns aqueous pastes prepared from dry converted starch, stabilized by being heated in paste form with limited proportions of a water-soluble carbamide-aldehyde condensate, triazine aldehyde condensate, or mixtures thereof.

This application is a continuation-in-part of our application Serial No. 300,173, filed October 19. 1939, issued Nov. 17, 1942, as United States Patent No. 2,302,310. In our'earlier application, there is described a method of stabilizing starch pastes with carbamide-aldehyde condensates with particular reference to starch pastes hydrolyzed in a wet way. The present application deals primarily with aqueous starch without regard to the particular method by which the paste is prepared and, specifically,

' with modlfied'starch pastes prepared from dry converted starch.

In the application of starch for coating or sizing of yarn, fabrics, paper, or other materials, it is necessary to treat the raw starch in order to obtain a material which will give a satisfactory deposition possessing the proper degree of such properties as penetration, adhesion, cohesion, plasticity, solubility, body, or other characteristic. It has been the practice to boil starches to produce viscous pastes or to treat starch chemically to obtain thin-boiling pastes of improved penetration. Hydrolysis of starches with the aid of an acidic substance or an enzyme produces pastes which have many highly desirable properties and by proper control of the process of hydrolysis it is possible to emphasize one set of properties in preference to another.

Pastes may be prepared directly from starch by wet processes, or they may lso be prepared by converting starch according dry methods, such as roasting, or heating with a trace of acid pastes of converted bility, or other physical characteristic or combination of properties, since the object of conversion is to give pastes of viscosities and solids contents which meet requirements of the many and varied applications of starch pastes. While the art has developed methods and means for producing starch pastes with the desired solids content, solubility, and body, there remains the problem of stabilizing the various pastes without altering their utilit for any intended application.

This problem is present whether the pastes are prepared directly in a wet way or are prepared by taking up with watera dry solubilized starch which has been obtained by dry conversion method or which has been prepared by solubilizing in a wet way followed by drying. The pastes tend to gel on standing or otherwise separate so that they are soon unfit for the intended use, Pastes also tend to putrefy on standing.

While dry converted starches prepared in the presence of acids, alkalies, urea, or oxidizing agents are not so economical as wet-processed starch pastes, and in some applications are less desirable, nevertheless there are applications where their convenience or peculiar properties recommend them for use, As long as the dry converted starches remain in the dry, powdered or other chemical reagent, Conversion or hydrolysis, both by wet methods and by dry methods, is today well known and commonly practiged. As a result of such conversion; starch is rendered soluble. The raw starch is changed to soluble starches, dextrins, and sugars, although in commercial practice sugars are generally present in definitely limited amounts. Extentof conversion is generally determined not by chemical change but rather by viscosity, soluform, there is no real problem as to preservation or stability. When, however, they are made into pastes, as is necessary in most applications, there arises the same necessity for stabilizing pastes made from dry solubilized starch as exists in the case of pastes made directly from starch by wet processes.

It is an object of this invention to provide a method for stabilizing partially hydrolyzed or converted starch pastes. It is an object to render such pastes highlyresistant to spoilage by molds or bacteria. It is an object to preserve starch pastes which have been partially liquefled by enzymatic action so that a smooth, homogeneous, stable product is obtained. It is also an object to render stable starch pastes which have been prepared by conversion of starch in other ways than with enzymes, including conver- "sion in the dry state. It is an object to increase the utility of converted'starch pastes by reaction of aqueous pastes with a water-soluble carbamide aldehyde addition or condensation product, particularly by the use of methylol or dimethylol urea or a methylol triazine.

It has been found that the partially hydrolyzed pastes may be stabilized by heating the hydrolyzed or converted products in paste form with a H or the binding of paper or other water-soluble carbamide or triazine-aldehyde reaction product. In general, a paste oi the desired degree'of hydrolysis or conversion is first prepared and then heated with. the carbamidealdehyde type reaction product. The paste then remains fluid, but viscous, homogeneous, stable against changes in these properties, and free from decay. a

For the preparation of converted starch pastes,

there may be used raw starch in a pure or crude i'orm. obtained from such sources as wheat, 'rye, corn, potatofsago, cassava, or the like. The starch is usually prepared in a powdered form and may readily be taken up with water to form a milk or' paste or, if it is in the form of lumps or granules, it may be mechanically worked with water until a slurry results. If enzymes are to be used for conversion, the starch slurry or suspension'is heated until the gelatinization point is reached. The temperature requiredior this varies with diiler ent starches and, in general, falls between 60 C. and 85 C. If; acid is used. for hydrolysis, the

elevated temperatures, generally 100 C. to about slurry'may' be similarly first heated, but this is:-

not essential as the acid hydrolysis may'start' usually well controlled in their properties. The

amylasesused may be obtained through the medium of fungi, glands, bacteria, or malt. Comparable results are obtained with enzymes from these different sources, although certain secondary differences in the resulting products are often of importance. In general, the procedure with enzymes is, as shown above, first to gelatinize the starch and then to allow the enzymes to react with the gelatinized starch until the desired de gree of body, viscosity, solubility, etc., is reached. At this point, the enzymes are deactivated by thermal or chemical means. In enzyme hydrolysis, the pH is of particular importance, and it is advisable to adjust the pH when necessary to the region of optimum activity for the particular enzyme used. Usually a pH between about 5.8 and 7.2 is desired and, if necessary, a small amount of alkali, acid, or buffer may be added to establish the best conditions. If deactivation is accomplished with a chemical, such as an acid, the step should be carefully controlled and excess acidity subsequently neutralized.

Hydrolysis of starch in the presence of acid is feasible and may be controlled, whether accomplished in wet or dry systems, to yield modified 1 starches or starch pastes useful for the sizing of yarns and fabrics, the coating of fibrous sheets,

materials together. Hydrolysis with acid bears a close resemblance. to, the procedure with enzymes but must be more critically controlled and requires acid-resisting apparatus. Organic or inorganic acids, such as hydrochloric, nitric, sulfuric, oxalic, acetic, etc., or mixtures of acids or acid-yielding salts, such as aluminum chloride, or oth'er chemical converting agent, may be used with control of temperature, concentration, and time of reaction, as is known in the art and as illustrated,

by way of example, in United States Patent; 579,- 827; 642,329; 642,330; 642,331; 675,822, or 742,469. When the desired stage oi conversion is reached, the agent is destroyed, as by neutralization. Ii a dry method is practiced,.starch may be roasted at 200 C., in the absence or presence of a converting agent, being agitated and heated until the desired degree 01' conversion is obtained. Such agent is usually an acid, which may be volatilized or consumed during the conversion or may be neutralized or inactivated when the required degree of conversion has been effected. It should be noted here that this invention is not critical as to the degree of conversion butis operable with the full range of pastes which are prepared by wet of starch for industrial ap- 1 a or dry conversion plications.

Insteadof any single type of converted starch,

mixtures of two or more-types may be prepared and handled inpaste'iorm.

'I'he concentration'of raw or converted starch ,usedin prep i s pas s mayb widely varied. In generaL'lJastes may be Prepared with Starch solids of 2% to 60%. Even higher solids contents may be obtained by gradual addition of starch during the process of hydrolysis or by carrying the dry conversion of starch to an advanced stage and taking up the dry converted starch in a limited amount of water, particularly with the aid of heat. Pastes may also be made from mixtures of converted and unconverted starch, as suchpastes are also stabilized by our method. Pastas containing low percentages of solids are useful in such operations as sizing, stiffening, and finishing of fabricsstarch, for example, are suitable for applications such as tub-sizing of paper. Pastes made from higher concentrations of starch are particularly .suitable in the beater sizing of paper, as adhesives for gumrned or coatedpaper, adhesives for envelopes, adhesives for laminating paper to paper, paper to cloth, etc., or binding fibers together as in paper board. The utility of pastes of the full range of concentrations'for the various applications of conventional starch pastes is retained when starch pastes are stabilized according to this invention by being mixed and heated with a water-soluble carbamide-aldehyde type condensate.

- The carbamide aldehyde type condensates which are useful in this invention are the watersoluble addition or condensation products obtained by reacting urea, thiourea, dlcyandiamide, a triazine such as melamine, thioammellne, fi,fibisthioammeline diethyl ether, or other carbamide type material, alone or in mixtures thereof with an aldehyde, particularly formaldehyde. or mixtures of aldehydes, such as acetaldehyde andformaldehyde. The preferred material is urea-formaldehyde in the form or a methylol urea. condensates of urea, melamine, and formaldehyde are also highly useful. The carbamide-aldeh'yde product is usually added after the starch has been carried to the desired stage of hydrolysis. On the other hand, when enzymes are usedfthe carbamide-aldehyde condensate may be added to the original slurry.

When a paste having the desired stage of hydrolysis is at hand, it is mixed with the .carbamide-aldehyde type condensate, and the resulting mixtures is heated for five to thirty minutes at 60C. or higher. Merely mixing starch paste and condensate fails to give stabilization.

Heating is essential. During the heating, there Pastes made with 2% to 15% of formaldehyde to one mol of melamine, refluxing the solution of reactants, and adjusting the solids content thereof to about 40%. The resulting mixture was heated at 75-80 C. for minutes and cooled. A stable converted starch paste was thus obtained.

Example 7 A paste having 35% solids was prepared by gelatinizing a mixture of corn and tapioca starches, hydrolyzing the gelatinized starches with a diastase of fungus origin at about 70 C., and inactivating the enzymes by heat. There was then stirred into the wet converted paste parts of a commercial dextrin to produce a paste of high solids content. When the paste was Example 8 A mixture of parts of a dry converted starch, 1.5 parts of a powdered, soluble urea-formaldehyde condensate which had been formed in solution in the presence of calcium chloride and dried, and 65 parts of water was stirred and heated to about 85 C. The resulting paste was a smooth one, which remained stable over a long period of time.

Examination of a series of starch pastes revealed the fact that the pH of all of these pastes,

whether made by enzyme conversion, by acid' hydrolysis in a wet way, or by dry conversion, was above 4.7. The addition of a carbamide-aldehyde condensate does not reduce the pH below this limit, but the addition of catalyst for hardening the condensate does. If desired, a catalyst such as ammonium thiocyanate, chloride, or phosphate, formic acid, acetic acid, or other acidic catalyst, may be added to the stabilized pastes just prior to application. The catalysts promote hardening of the carbamide-aldehyde reaction products and increase water resistance beyond that otherwise obtained from starch pastes,

The converted starch stabilized with urea formaldehyde as in the foregoing examples may, if desired, be bodied with borax. Starch pastes treated with borax alone are too stringy or too long for most purposes. But a paste treated with 1 to 10% of a urea-formaldehyde and then treated QAOQQQQ with a few per cent. of borax is stable, possesses goodtack and viscosity, and spreads well, being thus suitable as an envelope or box gum. It is, of course, possible, if desired, to modify starch pastes with gums and thickeners without destroying the stability of the pastes.

Pastes prepared as shown above are homogeneous and stable over a long period of time whereas the usual hydrolyzed pastes separate, become lumpy, stifl, and cracked, or otherwise undesirable within a few days. The method of stabilization herein described permits the preparation of pastes which are ready for use and which may be stored and shipped. They can be prepared with a viscosity and solids content suitable for any application where a dressing, sizing, stifiening, finishing, coating, or binding material is necessary. It thus becomes possible to prepare the widest variety of modified-starch pastes from a few basic ypes of starch with only a moderate degree of control.

In paper manufacture, the stabilized pastes may be used in beateror tub-sizing for coating or laminating paper, etc. In laundry work, they may replace the usual boiled starch to give smoother, more supple, yet amply stiff finishes. In textile manufacture, the stabilized pastes may be used for sizing warp or fabric, along with fillers;

such as clays, and with. such as tallow or soft res stiffening for special finishes, as in denims, glazes, embossed fabrics, etc. The pastes, particularly those of high solids content, are useful as adhesives for summed paper, envelopes, coatings, etc., and become quite water-resistant when dried, particularly under the influence of a catalyst and/or heat. The stab ed pastes, particularly those with the higher ratios of urea-formaldehyde to starch products, are useful binding agents for aqueous base prin n pastes- We claim:

1. The method of preparing a stable, modified starch paste which comprises making an aqueous paste of a water-soluble, partially hydrolyzed starch, adding to the paste and reacting therewith by means of heat between 0.5% and 25%, based on said starch, of a, water-soluble condensate of urea, melamine, and formaldehyde.

2. The product obtained by the method of claim 1.

or without softeners, for backfilling and SIVERT N. GLARUM. JOSEPH J. THOMAS. 

